This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to SV2000A000023 filed in Italy on Jun. 15, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to a process for manufacturing magnetic field generating devices in Nuclear Magnetic Resonance imaging apparatuses wherein the magnetic field is generated in a cavity or gap delimited by two pole pieces and wherein at least one pole piece includes a magnetized element, particularly a permanent magnet element and a ferromagnetic element, which is at least partly divided into individual modular elements arranged side-by-side in a predetermined order.
Several different processes and associated magnetic pole pieces as described above are currently known.
In accordance with a prior art process, the magnetic pole piece/s consist of a base element made of a solid and continuous material whereon an additional ferromagnetic layer is laid as a cover, which is formed by a plurality of blocks, each being in turn formed by a plurality of glued ferromagnetic sheets.
The purpose of this construction is to suppress or at least drastically reduce the formation of eddy currents induced in the ferromagnetic pole piece, which adversely affect the desired characteristics of the resulting magnetic field, and especially the magnetic field generated by gradient coils.
While the present laminate block construction at least partly solves the problem of suppressing currents in the ferromagnetic mass of the pole pieces, it heavily affects fabrication times and costs.
The invention has the object of providing a magnetic field generating process and device as described hereinbefore being simpler, faster and more cost-effective and allowing to obtain at least an equal effectiveness, or even an improved functionality in the suppression of eddy currents induced in the pole piece.
In accordance with a first process step, the invention proposes that each magnetic pole piece has an innermost layer of solid ferromagnetic material, i.e. made of one piece, and a layer composed of a plurality of adjacent or overlapping sheets, electrically insulated from each other.
The process also proposes that these sheets are mechanically pressed against and/or over each other.
As an additional improvement, the process proposes to provide mechanical means for engaging each sheet with the layer of solid ferromagnetic material.
A further step consists in forming a plurality of transverse notches or transverse apertures in the sheets in a comb-like arrangement, which are disposed at predetermined distances from each other and have a predetermined width, and extend from one face to the other of each sheet while, with reference to the sheet dimension parallel to the extension of the notch and/or aperture, said notches and/or apertures extend for a smaller length than the corresponding sheet dimension or in such a manner as to form thin bridges of material, for connecting the sheet areas which are separated by the notches and/or apertures.
The notches or apertures of the sheets may be coincident or non-coincident.
The notches may be all provided on the same edge of the sheet or be alternated on opposite edges.
Notches with different extensions may be alternated in any order.
Apertures, which may be provided alternatively to or in combination with notches may extend all at the same level and/or have all the same extension, or may be staggered in any predetermined order and/or have different extensions.
A further process step consists in assembling the layer of the pole piece which is composed of the sheets with notches and/or apertures and, after mechanically pressing the sheets to the end assembly condition, in eventually cutting the bridges of material which connect the sheet areas separated by the notches.
This cutting operation may be performed by any means. Particularly, this final cut may be performed by mechanical means and/or a laser beam.
According with a further variant embodiment, the process provides that the laminate layer of each pole piece is made of several metal foils laying over a one-piece or solid ferromagnetic plate and over each other, each foil being divided into a plurality of areas of predetermined shapes and sizes, by the provision of discontinuous notches interleaved with bridges of material for connecting the different areas delimited by the notches.
The notches are arranged in at least two transverse directions, whereas the bridges of material for connecting the areas separated by the notches are provided at least or only at the crossing points between the notches.
The individual ferromagnetic foils or sheets may have coincident notches or two sheets with coincident notches may have one or more interposed sheets whose notches are staggered with respect to those of the two sheets with coincident notches and/or to each other, thereby obtaining a pack of sheets with coincident notches alternated with one or more sheets with notches being staggered with respect to the former and or to each other.
A preferred embodiment provides a first set of sheets with coincident notches alternated with a second set of sheets having the same notch pattern as the first set with notches being coincident within the set and staggered by half the distance with respect to the adjacent notches of the first set.
Hence, the bridges of material provided at the crossing points between transverse notches overlap a median portion of the areas delimited by the notches of the underlying and/or overlying sheet.
The invention further provides bridges of material also at the peripheral edges of the sheets.
In the same manner as described above in relation to the first embodiment, the separation of the areas separated by notches may be completed by also cutting the bridges of material.
The bridges of material may be removed by mechanical punching and/or laser cutting. In this case, with reference to the preferred embodiment, the sheet areas separated by notches will have a central bore at the point overlapping the bridge of material of the underlying and/or overlying sheet.
Thanks to the above process, a magnetic pole piece with a modular and/or laminate structure may be fabricated in a simple, fast and cost-effective manner.
The notches or apertures limit the amount of material between the individual sheet areas, thereby drastically increasing the electrical resistance to the currents induced in the pole piece. When this conductivity limitation is insufficient, the final cut during assembly allows an easy and fast separation of the sheet areas separated by notches and/or apertures, thereby completely cutting off any electric contact between the individual sheet areas.
By performing the final cut when the modular part of the magnetic pole piece is in a substantially assembled condition greatly simplifies and speeds up the pole piece assembly operations, thereby reducing manufacturing costs.
A further improvement consists in selecting the orientation of notches or separation surfaces between sheets and/or areas of each sheet.
The currents induced in the pole piece appear to be closely related to the structure of gradient coils. In these conditions, advantages resulted from providing separation surfaces or notches between sheets and/or areas of each sheet oriented transverse to the conductors of the gradient coils.
Particularly, when the conductor/s only have one direction, the separation surfaces between sheets and/or the separation notches between the areas of each sheet are preferably oriented perpendicularly to the axis of the conductors, whereas when two conductors of the gradient coil cross each other or are angularly connected to each other, the separation surfaces and/or the notches should be oriented obliquely and as far as possible at the same distance from the two coil conductors.
The invention also relates to a magnetic field generating device fabricated in accordance with one or more of the process characteristics disclosed above.
Further improvements of the invention will form the subject of the subclaims.
The characteristics of the invention and the advantages derived therefrom will appear more clearly from the following description of a non limiting embodiment, illustrated in the annexed drawings, in which:
FIG. 1 is a schematic view of the structure of a magnetic field generating device in a Nuclear Magnetic Resonance imaging apparatus.
FIG. 2 is a perspective cutaway view of a pole piece of a device according to a first embodiment of the invention.
FIG. 3 is a transverse sectional view of the device of FIG. 2 with respect to a plane parallel to the sheets.
FIG. 4 is a transverse sectional view of the device of the previous figures with respect to a plane perpendicular to that of FIG. 3.
FIGS. 5A and 5B show a sheet according to two variant embodiments of the device of FIGS. 2 to 4.
FIG. 6 is a transverse sectional view of a device according to a second embodiment of the invention.
FIG. 7 shows a first variant of the device of FIG. 6.
FIG. 8 shows a second variant of the device of FIG. 6.
FIG. 9 shows the arrangement of two overlapping sheets with staggered notches in the two directions of the plane subtended by the sheets.
FIG. 10 shows an enlarged detail of the bridge of material connecting the sheet areas separated by notches.
FIG. 11 shows an enlarged detail like that of FIG. 10 in which 704xe2x80x2 denotes the point in which the punching means effect the separation.
FIG. 12 is a schematic view of the pattern of the notches separating the different areas of a sheet or of blocks of overlapping sheets as related to the orientation of the conductors of a gradient coil.